1. Field of the Invention
The present invention generally relates to data transmission in mobile communications systems and more particularly to sounding reference signal (SRS) transmission in carrier aggregation.
2. Description of the Related Art
In known wireless telecommunications systems, transmission equipment in a base station or access device transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an E-UTRAN (evolved universal terrestrial radio access network) node B (eNB), a base station or other systems and devices. Such advanced or next generation equipment is often referred to as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment is often referred to as an evolved packet system (EPS). An access device is any component, such as a traditional base station or an LTE eNB (Evolved Node B), that can provide a user agent (UA) such as user equipment (UE) with access to other components in a telecommunications system.
In mobile communication systems such as an E-UTRAN, the access device provides radio accesses to one or more UAs. The access device comprises a packet scheduler for allocating uplink (UL) and downlink (DL) data transmission resources among all the UAs communicating to the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between the UAs, deciding the resources (e.g. sub-carrier frequencies and timing) to be used for each UA's packet data transmission, and monitoring packet allocation and system load. The scheduler allocates physical layer resources for physical downlink shared channel (PDSCH) and physical uplink shared channel (PUSCH) data transmissions, and sends scheduling information to the UAs through a control channel. The UAs refer to the scheduling information for the timing, frequency, data block size, modulation and coding of uplink and downlink transmissions.
In certain communication standards, such as the 3GPP (3rd Generation Partnership Project) communication standard, carrier aggregation will be used for LTE-Advanced to support wider transmission bandwidths and hence increase the potential peak data rate to meet LTE-A requirements. In carrier aggregation, multiple (e.g., up to a maximum of five) uplink component carriers (CCs) may be aggregated, and they can be configured for use for a user equipment (UE). FIG. 1 shows an example of carrier aggregation. In this example, each component carrier has a bandwidth of 20 MHz, and the total uplink system bandwidth is thus 100 MHz. Note, however, that fewer than the five uplink CCs shown in FIG. 1 may be allocated to a particular UE and that the bandwidths of different CCs need not necessarily be the same. The UE may transmit on a multiple of up to five uplink CCs depending on the UE's capabilities. In addition, depending on the deployment scenario, carrier aggregation may include carriers located in the same frequency band and/or carriers located in non-adjacent (non-contiguous) frequency bands. For example, one carrier may be located at a 2 GHz band and a second carrier may be located at a 800 MHz band. Because each CC operates independently, each CC can be considered as one cell. For this reason, CC, cell and carrier can be considered as equivalent. More specifically, the primary carrier can be called as the PCC (primary component carrier) or the primary cell. In addition, the secondary carrier can be called as the SCC (secondary component carrier) or the secondary cell.
In a known release of the LTE specification (LTE Release-8 system), the eNB may configure the UE to transmit a sounding reference signal (SRS) in just one subframe or periodically in multiple subframes. The main purpose of SRS transmission is to help the eNB estimate the uplink channel quality to support frequency-selective uplink scheduling. In addition, SRS may also be used to control uplink power or uplink timing advance.
In this known release, an SRS is transmitted in the last single carrier frequency division multiple access (SC-FDMA) symbol in a subframe for both frequency division duplexing (FDD) and time division duplexing (TDD) as shown in FIG. 2. In addition, for TDD, SC-FDMA symbol(s) in Uplink Pilot Time Slot (UpPTS) may be used for SRS.
In a given cell, SRS from multiple UEs may be multiplexed in several domains. More specifically, the UEs may be multiplexed via code division multiplexing (CDM), time division multiplexing (TDM), fine frequency division multiplexing (FDM), and coarse FDM. With CDM, UEs using different cyclic shifts for SRS are multiplexed in a subframe. Eight different cyclic shifts nSRScs, are supported for SRS, which is defined in 3GPP, TS 36.211. With TDM, by allocating different periodicity and/or subframe offset, multiple UEs transmit SRS in different subframes. A SRS configuration index ISRS for SRS periodicity and SRS subframe offset Toffset are defined in 3GPP, TS 36.211. With fine FDM, the multiplexing uses a transmission comb across subcarriers. More specifically, with fine FDM, multiple UEs can transmit SRS on different sets of subcarriers (combs) in frequency domain; a transmission comb (kTC) is defined in 3GPP TS 36.211 and configured by higher layers. Since only a repetition factor of 2 is used in LTE, the set of possible values for kTC is {0, 1}. With coarse FDM, the multiplexing uses transmission bandwidth and frequency domain position. More specifically, different UEs can transmit SRS with different bandwidths and frequency domain location. The bandwidth and frequency domain position of SRS are configured by radio resource control (RRC) signaling. Because transmission of a large SRS bandwidth can require a larger transmit power compared to transmission of a narrow SRS bandwidth, a narrow bandwidth is preferable for cell-edge UEs. Due to this reason, each allowed configuration that is defined within the known release supports up to four different transmission bandwidth configurations, and the actual SRS bandwidth used for a transmission is dependent on both the configured cell specific SRS bandwidth parameter and the system bandwidth. Also, even if a small SRS bandwidth is configured for a UE, the eNB may be able to estimate the uplink channel quality of the entire bandwidth of this UE by using the frequency hopping of multiple SRS transmissions across multiple subframes. The parameters with respect to multiplexing are UE-specific parameters which are semi-statically configured by higher layers.
In the known release of the LTE specification, the eNB configures cell-specific SRS subframes and UE-specific SRS subframes. The cell-specific SRS configuration refers to SRS subframes reserved for potential SRS transmission from one or more UEs in a cell, while the UE-specific subframes indicate the subframes in which a particular UE should transmit SRS. Therefore, the cell-specific SRS subframe parameters are broadcast as system information, and the UE-specific SRS subframe parameters are signaled by dedicated RRC signaling to the particular UE.
Cell-specific SRS subframes are determined by the cell-specific subframe configuration period TSFC and the cell-specific subframe offset ΔSFC which are listed in Tables shown in FIGS. 3A and 3B, for frequency division duplex (FDD) and time division duplex (TDD), respectively.
The parameter srsSubframeConfiguration is the cell-specific SRS subframe configuration index parameter which is broadcast in system information. Sounding reference signal subframes are the subframes satisfying └ns/2┘ mod TSFCεΔSFC, where nS is the slot index (where there are two slots per subframe and ten subframes per radio frame, so 0≦nS≦19). For configurations where multiple values of ΔSFC are specified, SRS subframes are all the subframes satisfying the previous equation for all listed values of ΔSFC. For example, for srsSubframeConfiguration=13, subframes 0, 1, 2, 3, 4, 6 and 8 in each 10 ms radio frame will be reserved as cell-specific SRS subframes, but subframes 5, 7 and 9 will not be used for this purpose. For TDD, the sounding reference signal is transmitted only in configured uplink (UL) subframes or UpPTS.
The UE-specific SRS subframe configuration for SRS periodicity, TSRS, and SRS subframe offset, Toffset, is defined in the tables shown in FIG. 4A and FIG. 4B, for FDD and TDD, respectively. The SRS Configuration Index ISRS is configured by higher layers. The periodicity TSRS of the SRS transmission is selected from the set {2, 5, 10, 20, 40, 80, 160, 320} ms (or corresponding 1 ms subframes). For the SRS periodicity TSRS of 2 ms in TDD, two SRS resources are configured in a half-frame containing UL subframe(s).
In the known release of the LTE specification, to preserve the single carrier property, multiple uplink channels shall not be transmitted simultaneously in the same subframe. Therefore, when transmission of SRS and other uplink channels occurs in a same subframe, a multiplexing rule is applied. More specifically, the multiplexing rule sets forth: when PUSCH is multiplexed with SRS, SRS is transmitted in the last symbol of the subframe and the last symbol of PUSCH is not transmitted and accordingly, Uplink Shared CHannel (UL-SCH) data is rate-matched to the number of available symbols for PUSCH transmission. When ACK/NACK carried in PUCCH (Physical Uplink Control Channel) format 1a or format 1b is multiplexed with SRS, the method of multiplexing depends on the higher-layer configured cell-specific parameter Simultaneous-AN-and-SRS. If Simultaneous-AN-and-SRS is enabled, SRS is transmitted in the last symbol and ACK/NACK is multiplexed with SRS by using the shortened format of PUCCH 1a/1b, otherwise, SRS transmission is dropped and only ACK/NACK in PUCCH format 1a/1b (if present) is transmitted. When channel quality information (CQI) in PUCCH format2/2a/2b is multiplexed with SRS, since both CQI (non-aperiodic CQI) and SRS are periodic and semi-statically configured by the eNB, the UE should assume that simultaneous transmission of periodic CQI and SRS should not happen and such situation is due to an incorrect configuration, therefore, there is no defined UE procedure for this case in the specification. For aperiodic CQI transmission, as it is always associated with a PUSCH transmission, the rule for multiplexing PUSCH and SRS as described herein is applicable. Furthermore when positive SR (Scheduling Request) is multiplexed with SRS, the same rule as multiplexing ACK/NACK and SRS is applied.
For all cell-specific SRS subframes, all UEs transmitting PUCCH 1/1a/1b should not transmit on the last SC-FDMA symbol of such SRS subframe, regardless of whether or not a particular UE is configured to transmit SRS in that subframe. However, for the case of a configured cell-specific SRS subframe when a UE is transmitting PUSCH but is not transmitting SRS, that PUSCH will be transmitted on the last symbol of any PUSCH RBs which are not overlapped with resource blocks (RBs) belonging to a configured cell-specific SRS bandwidth. PUSCH is not transmitted on the last symbols of any RBs which overlap with cell-specific SRS RBs.
Because the independent SRS is configured per CC, a simple configuration approach would be to signal a distinct SRS Configuration Index for each uplink CC for UE specific SRS subframe. However, this parameter requires 10 bits (to signal values from 0 to 1023 as discussed herein), so the total overhead would be 10 bits (per UL CC) multiplied by the number of UL CC.
Also, in the known release of the LTE specification, the UE behavior is defined when SRS and other uplink channels are simultaneously transmitted. One philosophy behind these UE behaviors is to maintain the single carrier property, i.e. single carrier based approach. In carrier aggregation, the single carrier property could be relaxed because the UE supports multi-carrier transmission of PUSCHs. For this reason, simultaneous PUSCH and PUCCH transmission is supported within a CC or across multiple CCs in LTE-A. However, out of band emissions are still of concern with the multi channel transmission. Therefore, it is desirable to support both transmission approaches, the single carrier based transmission approach and the multiple channel simultaneous transmission approach. The approach that is actually supported is configured by the eNB or decided by the UE as UE capability.
In an LTE-A system, the simultaneous transmission of PUSCH and PUCCH is supported because the UE is capable of transmitting multiple channels with carrier aggregation. Similar to simultaneous PUCCH and PUSCH transmission, it is necessary to support the simultaneous transmission of SRS and other uplink channels, i.e., PUSCH and PUCCH. To maintain backward compatibility, in certain systems it would be desirable to apply the same approach as in the known release of LTE specification on a per CC basis when SRS and other uplink channels are simultaneously transmitted within one CC. Another remaining issue is when SRS and other uplink channels are transmitted simultaneously in different CCs.
If simultaneous transmission of SRS and other uplink channels is allowed, an issue is present relating to how to scale the power of each channel when the calculated total transmit power exceeds the maximum allowed transmit power, i.e., a power-limited situation. Compared to other uplink channels, SRS may have to be treated in a different way because the power-limited situation may only happen on the last symbol in which SRS is transmitted. FIG. 5 shows a case where a power-limited situation occurs. More specifically, at the beginning of the subframe, the transmit power is lower than the maximum power because only PUSCH is transmitted. But, in the last symbol, the transmit power is larger than the maximum power because of the additional SRS transmission on CC1 together with the PUSCH transmission on CC2.